1. Field of the Invention
The present invention relates to a radio access network apparatus and a mobile communication system using it, and more particularly to an improvement of a Radio Network Controller (RNC) in a W-CDMA cellular mobile communication system.
2. Description of the Prior Art
An architecture of a W-CDMA communication system that is a mobile communication system is shown in FIG. 11. A radio access network (RAN) 1 is configured with radio network controllers (RNC) 4, 5 and Nodes B6 to 9, and is connected with a core network (CN) 3 as an exchange network via an Iu interface. The Nodes B6 to 9 are logical nodes for radio transmission/reception, and more specifically radio base station apparatus.
An interface between the Node B and RNC is referred to as Iub, and Iur interface is also standardized as an interface between RNCs. Each Node B covers one or more cells 10 and is connected to a mobile unit (UE) 2 via a radio interface. The Node B terminates a radio line, and the RNC manages the Node B and selectively synthesizes radio paths in the case of soft handover. Note here that the detail of the architecture shown in FIG. 11 is specified in 3GPP (3rd Generation Partnership Projects).
FIG. 12 shows a protocol architecture for the radio interface in the W-CDMA communication system shown in FIG. 11. As shown in FIG. 12, such protocol architecture is composed of three protocol layers of a physical layer (PHY) 11 denoted as L1, data link layers 12 to 14 denoted as L2, and a network layer (RRC: Radio Resource Control) 15 denoted as L3.
The data link layers L2 are separated into three sublayers of a MAC (Media Access Control) layer 12, a RLC (Radio Link Control) layer 13, and a BMC (Broadcast/Multicast Control) layer 14. The MAC layer 12 has a MAC-c/sh (common/share) 121 and a MAC-d (dedicated) 122, and the RLC layer 13 has a plurality of RLCs 131 to 134.
Ellipses in FIG. 12 indicate service access points (SAP) between layers or sublayers, where SAPs between the RLC sublayer 13 and the MAC sublayer 12 provide logical channels. That is, the logical channels are provided from the MAC sublayer 12 to the RLC sublayer 13, and are classified by functions and logical properties of transmission signals and characterized by transferred information contents.
The logical channels include a CCCH (Common Control Channel), a PCCH (Paging Control Channel), a BCCH (Broadcast Control Channel), and a CTCH (Common Traffic Channel), for example.
A SAP between the MAC sublayer 12 and the physical layer 11 provides transport channels, which are provided from the physical layer 11 to the MAC sublayer 12. The transport channels are classified by a transmission form and are characterized depending on how and what information is transmitted via a radio interface.
The transport channels include a PCH (Paging Channel), a DCH (Dedicated Channel), a BCH (Broadcast Channel), and a FACH (Forward Access Channel), for example.
The physical layer 11 and the sublayers 12 to 14 in the data link layer are controlled by the network layer (RRC) 15 via a C-SAP providing a control channel. The detail of the protocol architecture shown in FIG. 12 is specified in TR25.925 of 3GPP.
In addition, a C (Control) plane for signaling that transfers a control signal and a U (User) plane that transfers user data, are specified in TR 25.925. The BMC sublayer 14 in L2 is applied only to the U plane.
The RNCs 4, 5 of the radio access network (RAN) 1 in the prior art are apparatus in which both functions of controlling the C plane and U plane are physically integrated.
In a mobile communication system including such a RNC that integrally has the control functions of both U plane and C plane, the control function of the C plane is sufficient enough to be added in order to enhance the signaling processing capacity, however, the RNC itself is required to be added. Furthermore, although the control function of the U plane is sufficient enough to be added in order to increase the transfer speed of user data, the RNC itself is required to be added as well. Therefore, the conventional RNC constitution makes constructing a system abundant in scalability quite difficult.
Moreover, the following disadvantage arises in soft handover. When a UE (mobile unit) is in a usual setup status, one Radio Link is connected between the RNC and Node B, and when the UE is moved and comes into a soft handover status, two or more paths are connected between the RNC and a plurality of Nodes B. When the UE comes into the soft handover across RNCs, a path is connected utilizing an interface referred to as Iur (see FIG. 11) between a serving RNC and a drift RNC.
In such a status of soft handover across RNCs, a path for user data may be connected from one U plane control function unit to a plurality of Nodes B involved in soft handover, however, another path for the user data needs to be connected between the serving RNC and the drift RNC, wasting resources and causing delay due to passing through the RNCs.